1. Field of the Invention
The present invention relates to a sheet conveying apparatus configured to convey a sheet while correcting skew of the sheet and an image forming apparatus configured to form an image on the sheet having the skew corrected, which includes the sheet conveying apparatus.
2. Description of the Related Art
Conventionally, electrophotography, offset printing, and inkjet printing are used in an image forming apparatus when forming an image on a sheet.
A color image forming apparatus using electrophotographic technology can be classified into a tandem-type color image forming system and a rotary-type color image forming system. The tandem-type color image forming system includes a plurality of image forming stations arranged in tandem while the rotary-type color image forming system includes a plurality of image forming stations arranged cylindrically. Further, the image forming apparatus can be classified according to a transfer system. In a direct transfer system, a toner image is directly transferred from a photosensitive drum onto a sheet. In an intermediate transfer system, a toner image is transferred once onto an intermediate transfer member before it is transferred onto a sheet.
A color image forming apparatus employs the intermediate transfer system having the tandem configuration in which four image forming stations that correspond to four colors are arranged along an intermediate transfer belt. In recent years, this type of color image forming apparatus has been introduced as a print on demand (POD) system aiming at a printing market of a small number of copies because such a system does not require a printing plate in forming an image on a sheet. However, high quality image output is required when the color image forming apparatus enters such a convenience printing market. In order to achieve the high quality, precise image forming registration (accuracy of transfer position of the image on a sheet) becomes important. Further, an image needs to be formed on a variety of sheets having different thickness. For example, an image needs to be formed on thin paper having approximate grammage of less than 50 g/m2.
Accuracy of image forming position is determined by factors such as registration in a sheet conveyance direction, registration in a direction orthogonal to the sheet conveyance, magnification, amount of skew, etc.
It is difficult to decrease an amount of skew by electrically correcting the skew. For example, it is certainly possible to detect skew, form an inclined image that corresponds to the skew, and form an image in a normal position. However, in a case of a color image forming apparatus that lays three or four colors to form an image, if the image is to be inclined in each sheet, dot forming of each color shows misregistration so that color tone varies on each sheet. Further, since time is required to calculate the amount of inclination of the image, productivity is greatly decreased.
Therefore, it is useful to eliminate skew by improving sheet conveying accuracy of a sheet conveying apparatus which conveys a sheet to a transfer unit that transfers a toner image onto the sheet.
The sheet conveying apparatus can employ the following three systems to improve the sheet conveying accuracy.
A first system employs a registration roller pair. In this system, the registration roller pair is arranged upstream of a transfer unit adapted to transfer a toner image onto a sheet. When the sheet is conveyed, the registration roller pair stops rotating and a leading edge of a sheet conveyed from the upstream is held at a nip of the registration roller pair. Since the sheet continues to be conveyed, the sheet deflects and skew is corrected. When the skew is corrected, the registration roller pair starts rotating and the sheet is conveyed in synchronization with the toner image. In this way, a toner image is formed on the skew-corrected sheet.
A second system employs a reference wall formed along a sheet conveyance direction and a skewed roller that causes the side edge of a sheet to hit the reference wall. The second system is described in U.S. Pat. No. 6,273,418. In this system, the sheet is guided to abut the reference wall by the skewed roller to correct the skew. Subsequently, a leading edge of the sheet is detected by a sensor and a conveyance speed of the sheet is controlled based on this detection. Thus, the sheet and the toner image are aligned, and a toner image is formed on the sheet in an appropriate position.
A third system employs two skew correction roller pairs. This system is described in U.S. Pat. No. 5,156,391. In this system, two skew correction roller pairs 426, 424 and 427, 425 are arranged in a direction orthogonal to the sheet conveyance direction as shown in FIG. 6A. The skew correction roller pairs 426, 424 and 427, 425 nip the sheet P while rotating independently to convey the sheet P. Conveyance roller pairs 436, 434 and 437, 435 which are arranged upstream of the two skew correction roller pairs, convey the sheet P to the skew correction roller pairs 426, 424 and 427, 425. Sensors 496 and 497, which are arranged downstream of the two skew correction roller pairs 426, 424 and 427, 425, detect an amount of skew of the sheet P.
Based on the amount of skew detected by the sensors 496 and 497, a conveyance speed of the skew correction roller pairs 426, 424 and 427, 425 is controlled so that the sheet P is rotated and the skew is corrected.
At the time the skew correction roller pairs 426, 424 and 427, 425 rotate the sheet P, if the conveyance roller pairs 436, 434 and 437, 435 arranged upstream of the two skew correction roller pairs nip the sheet P, the sheet P cannot be rotated. Therefore, as shown in FIG. 6B, before the skew correction roller pairs 426, 424 and 427, 425 rotate the sheet P, the skew correction roller pairs 426, 424 and 427, 425 slow down rotation speeds so that a deflection Pa of the sheet P is formed between the skew correction roller pairs 426, 424 and 427, 425 and the conveyance roller pairs 436, 434 and 437, 435. With this deflection Pa, a twist of the sheet P formed during its rotation can be allowed. Accordingly, the sheet P is rotated reliably and the skew can be corrected with improved accuracy.
However, in a sheet conveying apparatus employing the first or the second system, since the skew is corrected by using a stiffness of the sheet, it is difficult to correct skew of a thin sheet. That is, when a thin sheet abuts a nip of the registration roller pair or when a side edge of the sheet abuts the reference wall, the sheet is bent, which results in creases. Consequently, the skew cannot be reliably corrected.
A sheet conveying apparatus employing the third configuration forms a deflection by slowing down the skew correction roller pairs while the sheet is nipped by the skew correction roller pairs and the conveyance roller pairs. The deflection of the sheet is formed before the skew is corrected by the skew correction roller pairs. Accordingly, the conveyance speed of the sheet becomes slower when the sheet is deflected. Consequently, productivity (number of sheets output with an image formed in a certain period of time) is reduced.
Additionally, when in an attempt to reduce the size of such an image forming apparatus, a sensor which detects the amount of skew is placed upstream of the skew correction roller pairs so that the skew is corrected by the skew correction roller pairs directly after the amount of skew is detected, the following problem arises. Namely, in this case, the skew cannot be corrected until a deflection is formed between the skew correction roller pairs and the conveyance roller pairs. Accordingly, while the deflection is being formed, the sheet proceeds downstream so that a longer section (section L, see FIG. 6B) needs to be formed downstream of the skew correction roller pairs to correct the skew, and a longer space is required between the skew correction roller pairs and a transfer unit located downstream of the skew correction roller pairs. Therefore, the apparatus cannot be sufficiently down-sized.
As described above, the conventional sheet conveying apparatus employing the third configuration has a problem in productivity as well as downsizing since an image forming apparatus having such sheet conveying apparatus requires a longer space between the skew correction roller pairs and apparatuses located downstream of the skew correction roller pairs.